The present invention relates to a lens assembly and an electronic apparatus using the same. More specifically, the present invention relates to a device for transmitting an optical signal as the lens assembly and the like that are preferably applicable to any signal transmission between semiconductor chips such as large scale integrated (LSI) circuits.
Elements such as transistors and capacitors produced on a silicon substrate in an LSI chip at a front end thereof or blocks including such the elements have been wired and connected to each other at a back end thereof for transmitting a signal and supplying power by electric transmission using metal line produced on an insulating substrate. Accompanying, however, with speed-up in element operation based on current enhanced micromachining of scale, namely, high performance of a micro processing unit (MPU), a large amount of data, to be received in the chip, is necessary for being processed very rapidly, so that operation clock speed of the chip is remarkably increased.
Various issues have been arisen in metal wiring that electrically distributes the data and operation clock signal. As typical issues, have been risen any register and capacitor (RC) signal delay based on resistance and parasitic capacitance of the metal wiring, any impedance mismatching, electromagnetic/compatibility (EMC)/electromagnetic interference (EMI), any signal degradation due to cross talk or the like, any transmission error, increase of consumed power that is necessary for transmitting a signal based on any remarkable enhanced micromachining, increase of wiring length based on multilayer, deterioration of yield rate and the like.
As one of implementation techniques to solve such the issues, an optical interconnection between chips that LSI chips on a board are directly and optically connected to each other has been known. In order to realize such the optical interconnection between chips, an optical I/O package such that a beam output from a vertical cavity-surface emitting laser (hereinafter, referred to as “VCSEL”) is once collimated to focus at its incident to the waveguide has been proposed (see institute of electronics, information and communication engineers (IEICE) transaction C, pp. 793-799 (2001) Vol. J64-C, No. 9). FIG. 1 shows a configuration of a lens assembly, i.e., a device 500 for transmitting an optical signal as the optical I/O package. This device 500 has a VCSEL 511, as a light-emitting element, for diffusing an optical signal, a lens base 512 of VCSEL side, a collimator lens 513 for converting the optical signal diffused out of the VCSEL 511 from its diffused light into parallel light, a condenser lens 521 for gathering the parallel light output from the collimator lens 513 to focus the parallel light into an opening 531 of an optical waveguide 530, which is provided in an end of the optical waveguide 530, and a lens base 522 of waveguide side.
The optical signal (laser light) 540 diffused out of the VCSEL 511 is incident into the collimator lens 513, through the lens base 512, by which it is converted from its diffused light into the parallel light. The parallel light is also incident into the condenser lens 521 by which the parallel light is focused into the opening 531 of an optical waveguide 530, which is provided in an end of the optical waveguide 530, through the lens base 522. A mirror surface 532 having an inclined angle of 45 degrees then reflects the optical signal that has been focused into the opening 531. The waveguide 530 then guides the reflected optical signal along it.
According to the device 500 for transmitting the optical signal, for example, as shown in FIG. 2, even if a manufacture tolerance occurs and the waveguide 530 is shifted with respect to the VCSEL 511, a light path of the optical signal that is incident to the condenser lens 521 is also bended so that it is focused into the opening 531 of the waveguide 530, thereby configuring a lens module that is flexibly applicable to such the tolerance and has less transmission loss. It is to be noted that dotted line shown in FIG. 2 indicates positions of the condenser lens 521, the lens base 522, the waveguide 530 and the like if no manufacture tolerance occur.
The device 500 for transmitting the optical signal, shown in FIG. 1, can get sufficient coupling efficiency in transmission of the optical signal (laser light) from VCSEL 511 up to the opening 531 of the waveguide 530. If, however, perfect reflection of the optical signal that has been incident to the mirror surface 532 in the waveguide 530 is considered, it may be considered that a part of the optical signal passes through the mirror surface 532, not reflected by it, according to a shift direction of the parallel light that is incident to the condenser lens 521.
For example, it may be considered that, as shown in FIG. 3, occurrence of any manufacture tolerance causes parallel light that is incident to the condenser lens  521 to be shifted in a direction, “a” shown in FIG. 3 (that is, a reverse direction of a shifted direction, “b” shown in FIG. 2). In also this case, the device 500 can also get sufficient coupling efficiency in transmission of the optical signal (laser light) from the VCSEL 511 up to the opening 531 of the waveguide 530.
An incident angle of the optical signal, which has been focused into the waveguide 530, with respect to the mirror surface 532 becomes near 90 degrees with respect to this mirror surface 532. If a region in which the mirror surface 532 perfectly reflects the optical signal is estimated to αa as shown in FIG. 4, light that stays in a region αb, out of the region αa, fails to be reflected by the mirror surface 532 and passes through this mirror surface 532, so that the waveguide 530 cannot guide and transmit it. Thus, all the light within the region αb will be loss. In other words, an issue has been arisen such that when an incident angle of optical signal to the mirror surface 532 in the waveguide 530 is near 90 degrees, it is difficult to satisfy a perfect reflection condition by the mirror surface 532, thereby increasing loss in the amount of light.
Further, in order to decrease light that falls outside the region αa, it is estimated that NA of input light to the waveguide 530 is decreased. It is generally designed to make a light source and the mirror configured so as to become a conjugation relationship therebetween (i.e. to form an optical image thereon) in order to adjust angle shift in the light source, the collimator lens, and the waveguide. Herein, in order to decrease NA of the condenser lens 521, it is necessary to expand focal distance of the condenser lens 521.
Relationship between a focal distance of imaging lens and imaging factor is represented as following equation: imaging magnification=focal distance S2 of image side/focal distance s1 of object side (see FIG. 5). Thus, the focal distance of imaging lens (focal distance of image side) and the imaging magnification have proportionality relation so that the larger the focal distance of condenser lens 521, the imaging magnification of an image of light source in the mirror surface 532 becomes larger. In other words, if NA of the condenser lens 521 becomes smaller, a size of the image of light source (hereinafter, referred to as “focal diameter”) in the opening 531 of the waveguide 530 becomes larger.
FIG. 6 shows a case where NA of the condenser lens 521 becomes smaller. In this case, as shown in FIG. 7A, all the light of optical signal 540 stays within the region αa. In this case, however, NA of the condenser lens 521 becomes smaller, so that loss on amount of light occurs since the focal diameter Da fails to be included in the opening 531 of the waveguide 530. It is to be noted that a focal diameter Db shown in FIG. 7B indicates one relative to NA of the condenser lens 521 shown in FIG. 3.
Thus, if, in the device 500 for transmitting the optical signal, which has been shown in FIG. 1, NA of the condenser lens 521 becomes smaller in order to reduce the loss on the mirror surface 532 in the waveguide 530, an amount of light rejected at the opening 531 of the waveguide 530 increases while if NA of the condenser lens 521 becomes larger to make its imaging magnitude smaller in order to reduce an amount of light rejected at the opening 531 of the waveguide 530, an amount of the light passing through the mirror surface 532 increases without performing any perfect reflection on the mirror surface 532 when any manufacture tolerance occurs.